At standstill after a previous operation, an internal combustion engine heats its surroundings whereby fuel-conducting parts such as injection valves and lines are especially affected. Vapor bubbles can then form and lead to an inadequate supply of the engine with fuel when the engine is restarted; that is, the vapor bubbles can lead to an unwanted leaning of the mixture. A poor startup and idle performance results as a consequence. Relationships between hot-start conditions and the formation of vapor bubbles in the fuel system are disclosed, for example, in U.S. Pat. No. 4,951,633.
Known methods provide a compensation of the unwanted leaning of the mixture by means of a controlled enrichment in dependence upon the temperatures of the engine and the intake air. This enrichment is reduced in a controlled manner and finally set to zero in dependence upon the elapse of time from the hot start.
These measures are primarily suitable to compensate for a leaning of the fuel mixture occurring for a short time after a start. The term "short term" is intended here to be a time interval in the order of magnitude of one minute. Furthermore, the stabilization of the idle operation above an increased idle rpm is known.
The amount and the time duration of the unwanted leaning of the mixture by the formation of vapor bubbles is influenced, on the one hand, by the geometry of the arrangement of the engine and the fuel-metering parts and, on the other hand, by the quality of the fuel used.
An especially critical geometry is present, for example, in a V-engine wherein the injection valves and the feed lines (fuel rail) lie between the two cylinder banks and are covered from above by the intake pipe. With this arrangement, the heat transfer to the fuel rail is facilitated while at the same time the cooling of the fuel rails by convection is hindered.
Experiments show that especially under these conditions after a hot start even an intense leaning of the mixture exceeding lambda=1.5 occurs over the time interval of several minutes which is accompanied by intense fluctuations in the value of lambda.
The controlled enrichment according to the state of the art cannot completely compensate the disturbances in mixture because of the extent of the vaporization performance of possible types of fuel available.